Corrosion occurs when metals are oxidized to their respective ions and/or insoluble salts. For example, corrosion of metallic iron can involve conversion to soluble iron in a +2 or +3 oxidation state or insoluble iron oxides and hydroxides. Also, corrosion has a dual nature in that a portion of the metal surface is removed, while the formation of insoluble salts contributes to the buildup of deposits. Losses of metal cause deterioration of the structural integrity of the system. Eventually leakage between the water system and process streams can occur.
Corrosion of iron in oxygenated waters is known to occur by the following coupled electrochemical processes: ##STR3##
Inhibition of metal corrosion by oxygenated waters typically involves the formation of protective barriers on the metal surface. These barriers prevent oxygen from reaching the metal surface and causing metal oxidation. In order to function as a corrosion inhibitor, a chemical additive must faciliate this process such that an oxygen-impermeable barrier is formed and maintained. This can be done by interaction with either the cathodic or anodic half-cell reaction.
Inhibitors can interact with the anodic reaction (1) by causing the resultant Fe.sup.+2 to form an impermeable barrier, stifling further corrosion. This can be accomplished by including ingredients in the inhibitor compound which:
React directly with Fe.sup.+2 causing it to precipitate; Facilitate the oxidation of Fe.sup.+2 to Fe.sup.+3, compounds of which are typically less soluble; or, Promote the formation of insoluble Fe.sup.+3 compounds.
The reduction of oxygen at corrosion cathodes provides another means by which inhibitors can act. Reaction 2 represents the half cell in which oxygen is reduced during the corrosion process. The product of this reaction is the hydroxyl (OH.sup.-) ion. Because of this production of hydroxyl, the pH at the surface of metals undergoing oxygen mediated corrosion is generally much higher than that of the surrounding medium. Many compounds are less soluble at elevated pH's. These compounds can precipitate at corrosion cathodes and act as effective inhibitors of corrosion if their precipitated form is impervious to oxygen and is electrically nonconductive.